Photochromic materials react to light levels by darkening in sunlight (or other light having a UV component), and spontaneously reverting to a faded state under low light conditions, or indoors where the UV light is removed. An example of a photochromic material is found in the TRANSITIONS™ lenses. U.S. Pat. No. 6,065,836 describes a photochromic opthalmic lens with a film comprising a photochromic dye. Both fading and darkening are ‘passive’, in that they occur without input of a user, or in a controllable manner. Such materials may be less useful in an environment where UV is lacking, or variable, or where temperature fluctuates. Such photochromic eyewear worn inside a vehicle on a bright day may have the darkening reaction inhibited by the low amount of UV light in the vehicle (e.g. caused by UV blocking PVB layer of the windshield). Conversely, the glasses may go dark when a user does not wish them to.
Thermochromic materials respond to temperature—US 2009/0167971 describes an optical filter comprising a liquid crystal material that is thermochromic—when used as a window film, it may be useful in attenuating heat gain in a building under some conditions, but may not be controllable to provide full light transmission under higher temperature conditions. The temperature dependent darkening and fading is passive, and may occur without the input of a user, or in a controllable manner. Such materials may be less useful where temperature fluctuates, where a dark state is desired in a low temperature environment, or where a faded state in a high temperature environment is desired.
Electrochromic materials respond to application of electricity. Some electrochromic materials (e.g. bipyridinium-group containing materials) darken with application of a voltage, but fade passively when the application ceases. Such materials may employ a conductive layer such at ITO to effect operation—where these materials are used as an overlay or underlay for some configurations of displays (e.g. touchscreens), operation of the electrochromic material (e.g. providing an electric voltage to the conductive layer of the electrochromic material) may interfere with operation of a touchscreen. The applied voltage to induce the color change may vary with the nature of the material, from a few volts to as much as 60V or 120V to effect the transition between dark and faded states. Some electrochromic materials may require continuous application of electricity to maintain their dark or faded states.
Some switchable materials may provide controllable darkening, such as electrochromic, liquid crystal or suspended particle materials, but may have other disadvantages such as cost, lack of optical clarity, reduced lifetime or haze, or may be difficult to adapt to some applications (e.g. incorporation into safety glass by lamination with PVB).
Controllable darkening may be useful not only for glazings or opthalmic applications, but for reducing or blocking light from displays. In some applications, it may be desirable for the display to be ‘blacked out’ when not in use. Some embodiments may overlay a device with a tinted glass with a low visible light transmittance to appear black when not in use. When it is desired to view the display or interact with the touchpad, the display screen behind the tinted glass is illuminated with a light source to visualize icons, images or the like on the display. Depending on the degree of tint of the tinted glass, the luminance necessary to make the display visible to a user may be substantial. In some applications, the light source providing a suitable level of luminance may represent a considerable power draw, and/or generate excessive heat. If the display is in a confined space (e.g. a steering column or dashboard of a vehicle, control panel of an aircraft, or the like), or used in a device that must carry or supply its own power (e.g. battery operated), the heat production and/or power draw may represent a substantial engineering obstacle. Further, excessive heat production in a confined space may affect the operation of other instruments or device, such as computer controls.